Methods for radiolabelling human leucocytes with gamma emitters have been reviewed by H. J. Danpure and S. Osman ("A Review of Methods of Separation and Radiolabelling Human Leucocytes," Nuclear Medicine Communications (1988); vol. 9, 681-685). Danpure and Osman indicate that successful radiolabelling methods employ neutral lipophilic agents which chelate the selected gamma emitting radionuclide and carry the radionuclide across the cellular membrane into the patient's or donor's isolated leucocytes. Once inside the leucocyte, the lipophilic chelator may be degraded or otherwise modified so as to produce a hydrophilic radionuclide product. The hydrophilic radionuclide product remains entrapped within the leucocytes for a period sufficiently long to allow rapid injection of the labelled leucocytes into the patient, migration of the labelled leucocytes to inflamation sites, and scintigraphic imaging of inflamation sites by means of the labelled leucocytes. Danpure and Osman indicate that the gamma-emitting radionuclide Indium-111 oxine has been successfully employed in clinical settings for radiolabelling mixed leucocytes for scintigraphic imaging.
As an alternative to Indium-111 oxine, A. M. Peters et al. introduced Tc-99m d,1-HMPAO (Technetium-99m-d,1-hexamethylpropylene-amine oxime) for radiolabelling leucocytes ("Preliminary Clinical Experience with Tc-99m d,1-HMPAO for Labelling Leucocytes and Imaging Infection," Lancet (1986); vol. 11, 945-949) Technetium-99m is a widely available generator-produced radionuclide ideally suited for gamma detection. Use of Tc-99m d,1-HMPAO can produce a good scintigraphic image at a relatively low radiation dosage. The conversion of Tc-99m d,1-HMPAO from a lipophilic form to a hydrophilic form has been characterized by Joseph C. Hung et al. ("Kinetic Analysis of Technetium-99m d,1-HM-PAO Decomposition in Aqueous Media," Journal of Nuclear Medicine (1988); vol. 29, 1568-1576).
An advanced method for labelling leucocytes with Technetium-99m d,1-HM-PAO has been disclosed by N. N. Solanki et al. ("A Rapid Method for the Preparation of TC99-m Hexametazine-Labelled Leucocytes," Nuclear Medicine Communications (1988); vol. 9, 753-761). Solanki et al. indicate that leucocytes are best separated from whole blood by a 1.times.g sedimentation followed by a gentle (100.times.g) centrifugation of the resultant leucocyte platelet-rich plasma (LPRP). The resultant pellet is then re-suspended in 10% cell free plasma (CFP) and incubated with Tc-99m d,1-HMPAO. Solanki et al. claim a labelling efficiency of 25.9% when employing the recommended reconstitution volume and a labelling efficiency of 49% when employing a low reconstitution volume.
Achieving a labelling efficiency in excess of 25.9% with Tc-99m d,1-HMPAO while employing the recommended reconstitution volume is desirable since enhanced labelling efficiency reduces the size of the blood sample which must be drawn from the patient, enhances the resultant image quality, and decreases the amount of Technetium-99m d,1-HM-PAO reagent which must be employed, and minimizes contamination by red blood cells (RBCs).